1. Field of the Invention
The present invention generally relates to a knock control system that suppresses knocking in a spark ignition type of internal combustion engine. More specifically, the present invention relates to a knock control system used in an internal combustion engine that is equipped with a variable valve mechanism that changes the closing timing of the intake valves.
2. Background Information
A knock control system is disclosed in Japanese Laid-Open Patent Publication No. 2000-205096 that suppresses the occurrence of knocking in a spark ignition internal combustion engine that compresses a mixture of air and fuel, and then ignites the mixture with a spark plug. In a well-known manner, the knock control system uses feedback-control to adjust the ignition timing based on a knock signal of a prescribed level outputted from a knock sensor.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved knock control system. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
It has been discovered that, in the above mentioned knock control system, if the signal-to-noise ratio S/N is poor, there is the risk that knocking will be detected incorrectly when knocking is not occurring or that knocking will fail to be detected when knocking is actually occurring. In particular, when the intake valve closing timing overlaps with the knock detection interval, the signal-to-noise ratio S/N degrades considerably due to vibrations and noise that occur when the intake valve returns to the seated position. Furthermore, in an internal combustion engine equipped with a variable valve mechanism that can vary the closing timing of the intake valves, the degree of overlap between the intake valve closing timing and the knock detection interval changes depending on the intake valve closing timing.
The present invention was conceived in view of these problems. One object of the present invention is to improve fuel efficiency and exhaust performance by precisely determining situations in which the knock signal cannot be accurately and easily detected, i.e., situations in which knock control cannot be executed, and reducing as much as possible the occurrence of situations in which knock control cannot be executed.
In accordance with one aspect of the present invention, a knock control system for an internal combustion engine is provided that comprises a variable valve mechanism, a knock detector, a signal-to-noise ratio estimating section, an estimated noise signal updating section, a knock detectability determining section, and a knock control section. The variable valve mechanism is configured and arranged to change at least an intake valve closing timing. The knock detector is configured to output a knock signal indicative of the engine knocking condition during a knock detection interval. The signal-to-noise ratio estimating section is configured to estimate a signal ratio based on at least one of an engine rotational speed and an engine load. The signal ratio is a ratio of an estimated knocking signal estimated when the engine knocking condition is occurring and an estimated noise signal estimated when the engine knocking condition is not occurring. The estimated noise signal updating section is configured to update the estimated noise signal depending upon on the intake valve closing timing. The knock detectability determining section is configured to determine if the knock signal can be detected or not based on the signal ratio. The knock control section is configured to execute knock control based on the knock signal when the knock detectability determining section determines the knock signal can be detected.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.